In recent years, semiconductor wafers (hereinafter, referred to as “wafers”) are increasing in their diameter in manufacturing semiconductor devices or micro electromechanical systems (MEMSs). In addition, there is a desire to make wafers thin in a specified process such as mounting or the like. However, a large-diameter wafer is likely to be bent or cracked if the wafer is carried or polished as it is. Therefore, in order to reinforce the wafer, the wafer is bonded to a reinforcing substrate, for example, another wafer having the same shape and size as the wafer. Typically, the two wafers are bonded together using an adhesive.
However, such bonding between the two wafers using the adhesive is likely to generate voids due to air introduced between the two wafers. Such voids generated between the two wafers may deteriorate performance of semiconductor devices to be manufactured and, therefore, there is a need to suppress the generation of voids.
To overcome this problem, in a related art, there has been proposed a bonding apparatus including a chamber which accommodates two wafers vertically arranged (hereinafter, a wafer in an upper side is referred to as an “upper wafer” and a wafer in a lower side is referred to as a “lower wafer”), pressing pins which are provided within the chamber and press the center of the upper wafer, and a spacer which supports the outer periphery of the upper wafer and can be moved back from the outer periphery of the upper wafer.
In this bonding apparatus, in order to prevent voids from occurring between the two wafers, the wafers are bonded together with the interior of the chamber exhausted and decompressed. Such bonding between the wafers is performed by first pressing the center of the upper wafer by means of the pressing pins with the upper wafer supported by the spacer, contacting the center to the lower wafer, moving the spacer supporting the upper wafer back from the upper wafer, and then contacting the entire surface of the upper wafer to the entire surface of the lower wafer.
However, this bonding apparatus requires the chamber to be under a decompressed state during the entire bonding process. This process consumes much time from the time the wafers are moved into the chamber to the exhaustion of the entire interior of the chamber and the decompression of the chamber to a predetermined pressure, which results in a low throughput of wafer processing.
Therefore, there has been proposed a technique in which the center of an upper chuck holding an upper wafer is bent with a predetermined pressure by exhausting the interior of a narrow sealed space formed by making close contact between the upper chuck and a lower chuck, thereby first contacting the center of the upper wafer with the center of a reinforcing substrate or the lower wafer, and then a bonding process is performed by contacting both wafers together sequentially from their centers to their outsides in a radial direction.
In the above-described wafer bonding processes, the wafers are bonded together with the adhesive melted into a liquid state by heating the adhesive to its melting point or above. Accordingly, for example, a heater as a heating mechanism is contained in the lower chuck and the adhesive applied on the lower wafer is heated by the heater.
However, the present inventors have performed the wafer bonding process as known in the art, and have discovered that there are some cases where voids occur between the wafers even in the case of bonding the wafers in the decompressed space.
The present inventors have examined this discovery and have found that the voids are attributed to thermal expansion of the lower chuck due to heating by the heater. In more detail, as shown in FIG. 15, the periphery of a lower chuck 101 embedded with a heater 100 is supported by an annular support 102 made of, for example, stainless steel. The lower chuck 101 and the support 102 are fixed by a plurality of bolts 103 concentrically arranged. The support 102 is fixed to a pedestal (not shown) by means of a plurality of other bolts.
It has been observed that the outer periphery of the top of the lower chuck 101 is deformed into a wavy state when the temperature of the heater 100 is increased. In this regard, the present inventors have calculated, through a simulation using numerical analysis, a degree of displacement of the outer periphery of the top of the lower chuck 101 when the temperature of the heater is increased to 210 degrees Celsius and have obtained the results shown in the table of FIG. 16. This simulation can use universal software for thermo structure analysis. In FIG. 16, X1 to X12 shown in the left side of the table represent measuring points of the degree of displacement and numerical values shown in the right side of the table represent the degree of displacement (in the unit of μm) in the vertical direction at the measuring points. The lower chuck 101 shown in FIG. 16 is fixed to the support 102 by means of six (6) bolts 103.
It can be seen from this result that a degree of displacement due to thermal expansion at positions corresponding to the bolts 103 is relatively small, whereas a degree of displacement is increased by a maximum of 30 μm at positions between the bolts 103, as compared to the positions corresponding to the bolts 103. The reason why the degree of displacement due to thermal expansion at the positions corresponding to the bolts 103 decreases is that the extension of the lower chuck 101 due to thermal expansion is restrained by the bolts 103. Accordingly, it is believed that the outer periphery of the top of the lower chuck 101 is deformed into a wavy state. As a result, it is inferred that airtightness by close adhesion of the upper chuck to the lower chuck at the positions corresponding to the bolts 103 is reduced and there is a high possibility of generating voids in the vicinity of the positions.